1. Field of the Invention
The present invention relates to twisted differential pair electrical conductors on circuit boards.
2. Background Art
Differential signaling is a technique for transmitting information electrically in the form of a pair of complementary signals transmitted on separate conductors. Twisted differential pairs may be used in differential signal transmission over long transmission lines, where a significant amount of crosstalk in differential mode may exist between adjacent differential pairs. In a twisted differential pair, the two conductors are twisted around each other such that the two conductors more closely receive the same interference. The pitch of the twists (distance between adjacent twists) is typically configured such that desired signal parameters in differential mode, such as rise time, are not compromised significantly, while minimizing the cross-talk between adjacent pairs due to the reduced differential loop areas and the location of the cross-overs of the twists. Also, EMI radiation from the differential loops formed in a twisted differential pair by the twisting is reduced due to the reduced loop areas.
Such twisted differential pairs are used in cables and wires, and may also be implemented in long traces on circuit boards to achieve the above described performance benefits. Current techniques for implementing a twisted differential pair on a circuit board require more than one metal layer for routing the signal traces to facilitate the cross-over between the traces. The two conductors of the conductor pair are routed on a first metal layer of the circuit board, and the cross-over routing is formed on a second metal layer of the circuit board. Vias through the circuit board are used to connect the conductor pair with the cross-over routing between the first and second metal layers.
Such conventional techniques for implementing twisted differential pairs in circuit boards have numerous deficiencies. For instance, the requirement for more than one metal layer typically results in a cost increase for the circuit board. Furthermore, the requirement for vias through the circuit board can result in associated performance issues, such as impedance mismatches due to the increased capacitances at the via pads, and may cause routing density issues.